For me, the issue was with my proxy_pass entry. I had
location / {
…
proxy_pass http://localhost:5001;
}
This caused the upstream request to use the IP4 localhost IP or the IP6 localhost IP, but every now and again, it would use the localhost DNS without the port number resulting in the upstream error as seen in the logs below.
[27/Sep/2018:16:23:37 +0100] <request IP> – – – <requested URI> to: [::1]:5001: GET /api/hc response_status 200
[27/Sep/2018:16:24:37 +0100] <request IP> – – – <requested URI> to: 127.0.0.1:5001: GET /api/hc response_status 200
[27/Sep/2018:16:25:38 +0100] <request IP> – – – <requested URI> to: localhost: GET /api/hc response_status 502
[27/Sep/2018:16:26:37 +0100] <request IP> – – – <requested URI> to: 127.0.0.1:5001: GET /api/hc response_status 200
[27/Sep/2018:16:27:37 +0100] <request IP> – – – <requested URI> to: [::1]:5001: GET /api/hc response_status 200
As you can see, I get a 502 status for “localhost:”
Changing my proxy_pass to 127.0.0.1:5001 means that all requests now use IP4 with a port.
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